A converter, and more precisely a voltage source converter provides the electrical coupling between a DC voltage system and an AC voltage system comprising one or a plurality of phases. Depending on the power direction, a converter has either the function of a rectifier, which delivers electric power from the AC system to the DC system, or an inverter, which delivers electric power from the DC system to the AC system. By way of example, a converter may be used for variable-speed control of a synchronous or asynchronous rotating machine as well as transmission of high voltage direct current (HVDC) over long distances.
The simplest converter comprises a two level bridge composed of two valves. Each valve comprises a single or a plurality of switches. A three phase converter thus comprises a bridge with six valves where each valve comprises at least one switch. A switch comprises a turn-off device and a diode in antiparallel connection therewith. By this arrangement the current is controllable stopped in one direction but freely passing in the opposite direction. For high voltage applications each valve comprises a plurality of series connected switches with such turn-off devices and antiparallel diodes.
Since the load is of the inductive type, it is necessary for a diode referred to as a “free-wheeling diode” to be placed in parallel with the switch in order to allow the load current to flow when the corresponding switch is open. A further development of the two level converter is the three level converter which requires six extra diodes. This converter is also known as a neutral point clamped (NPC) converter bridge.
Using one bridge of a two-level converter as an example the AC output voltage of a converter, the amplitude, the phase angle and the frequency of the fundamental frequency as well as the harmonic distortion, is controlled by alternatively switching on and off the two valves on the bridge connected to the same phase. Thereby, the AC current is controlled as desired. The pulse signals for controlling the switches are generated according to a selected Pulse Width Modulation (PWM) method.
There are a large variety of PWM methods. Most often used methods are carrier based PWM, such as Sinusoidal Pulse Width Modulation, SPWM, and carrierless PWM, such as Optimum Pulse Width Modulation, OPWM. The modulation techniques in the prior art are based on the assumption that the switching elements of the converter operate in an ideal manner, that is, they switch on or off exactly at the instants the control dictates. These are reckoned as ideal switching instants in the following text. In reality, however, the converter output voltage waveform deviates from what the control originally dictated.
A first reason is that the switching devices are not ideal. A switching device has a delayed reaction to its control signal at a turn-on and a turn-off switching respectively. The delayed reaction depends on the type of semiconductor, on its current and voltage rating, on the controlling waveforms at the gate electrode, on the device temperature, and in particularly on the actual current to be switched.
A second reason is the blanking time, or “dead time”, which must be inserted between an opening (turn-off) order of a first valve and a closing (turn-on) order of a second valve on the same bridge. The presence of a blanking time is causing the two valves of a converter bridge never to be closed at the same time in order to prevent a short-circuit.
A third reason which contributes to the deformation of the output voltage is the difference in the rising and descending rate, dv/dt, of the voltage across the switch devices during turn-off and turn-on. This may be due to the existence of a snubber circuit or parasitic capacitance in the diodes. The deformation is noticeable in particular when the switching current is low.
According to the reasons mentioned there will be a delay between the switching order and the actual switching event. In order to achieve an actual switching event that correspond to the ideal switching instant the switching order must be sent in advance. Thus for every switching there must be taken into consideration an action time of the valve. The action time of a valve is in the following text defined as the time difference between the actual switching order and its actual switching event. Thus the action time comprises the delayed reaction of the switching device, the blanking time and the variation due to the low rising and descending rate of the voltage (dv/dt). The consequence of the variation of these parameters is giving rise to an non-linear error between the commanded voltage and the real converter output voltage. This results not only in additional low order of harmonics, for instance, 5th and 7th harmonics, but sometimes also in instability problems of the control system. Therefore, there have been many attempts made to correct or to compensate these errors.
From U.S. Pat. No. 5,991,176 a method for processing PWM waves and a device therefore is previously known. The object of the method is to reduce or eliminate the effect of the blanking time (referred as dead time) in an inverter or a controlled rectifier. The known inverter is controlled by a modulator and a discriminator. The role of the modulator is to create a set wave, whereas the discriminator makes it possible to split this wave into a plurality of waves which are intended for individually controlling the various switches. The purpose of the discriminator is to introduce a delay on the closing of the corresponding switches, so that it is always certain that, when the command to close one switch is given, the opposite switch is already open.
The known method suggest the use of two corrected control set signals, one for the case when the current is an output current and one for the case when the current is an input current. It is the direction of the current in the load which will determine whether one or the other of the two corrected set signals is to be used. Thus the switching order is compensated for the blanking time.
From U.S. Pat. No. 6,535,402 a method for adaptive compensation of dead time for an inverter and a converter is previously known. The object of the method is to compensate the effect of the dead time to avoid current distortion and torque ripple in motors driven by such an inverter. The document appreciates the difficulty to measure the zero crossing of the current and thus propose a bias current applied on the distorted current. Then it is established when the current passes the bias level of the current. A second dead time compensation is derived from the current crossing of the bias level and added to a first dead time compensation of a PWM signal.
The known methods for correcting the error between the commanded voltage and the real output voltage in the prior art are based on current measurements. Thus the known methods are based on measurement of current switching. A feed-forward type compensation is provided which corrects only the average voltage error due to the blanking time or low dv/dt. The error due to the reaction time of the switching devices is not considered. There is no feedback control or confirmation to tell if the turn-on or turn-off of the switching devices occur at the exact instant that control dictates. In addition, the method described in U.S. Pat. No. 6,535,402 requires additional hardware components, which can be very costly in high power application.
The methods known from the prior art may work well enough in some conditions. In other conditions they may fail to function properly. One such case is when the switching frequency is low and the inductance is also low, which may result in a very high current ripple. Typically in high power applications of STATCOM and HVDC the converters are directly connected to the grid. In such situations they will have high switching current ripple. It is obvious in such a case that the current direction is different from one switching instant to a next switching instant.
It may be possible to use a predicted the current at the next switching instant to estimate the action time for next switching in advance. However, it is very difficult to guarantee the correctness of the predicted current, as the correctness of the predicted current relies not only on the converter reference voltage, on the accuracy of measured current and measured voltage, but also on the calculation speed of the control process.
In high power applications, such as HVDC and STATCOM, the low order harmonics lead to very high cost for filtering apparatus. Thus, there is a need for a new control method, which can realize high precision switching thereby eliminates the effect of errors mentioned above, for voltage source converters in high power application.